1. Field of the Invention
The present invention relates to a novel scheme which minimizes fuel consumption in thermally cracking a hydrocarbon feedstock and separating the cracked product. In particular, it relates to thermal cracking of a hydrocarbon feedstock in the presence of steam at temperatures of about 1200.degree. to 1800.degree. F. More specifically, it pertains to preheating combustion air, before it is blown into the thermal cracking furnace, in a heat exchanger by employing bottom pumparound (BPA), top pumparound (TPA) and/or quench water (QW) streams extracted from the primary fractionator which is externally connected to the pyrolysis tubular metal reactor located within the furnace. The heat transferred at low temperatures to the combustion air becomes available above the unheated fuel adiabatic flame temperature for transfer to the furnace tubular reactor.
2. Description of the Prior Art
Since the thermal efficiency of a pyrolysis reactor furnace depends on how much of the thermal energy released from the fuel has been absorbed and utilized within the furnace, efforts have been made to lower the temperature of the combusted flue gas leaving the furnace, thereby maximizing the recovery of the fuel energy. One approach towards reducing the flue gas temperature has been to use the flue gas to preheat the combustion air used in the furnace burners. This recovers heat from the flue gas and improves the overall thermal efficiency of the furnace. The concept of preheating the combustion air with the flue gas stream has been extensively studied.
Unfortunately, however, utilization of the flue gas in preheating the combustion air is attended by several inherent engineering disadvantages. First of all, it requires a high investment for the installation of blowers, drivers, insulated ducts and other miscellaneous equipment needed to transport the hot flue gas to a heat exchanger wherein heat transfer between the flue gas and the combustion air takes place. Further, the heat exchanger and part of the flue gas transportation equipment are vulnerable to corrosion as they are in direct contact with acidic components of the cooled flue gas. Finally, the regenerative heat exchanger normally employed for this is subject to outages which deleteriously affect the furnace service factor.
Another approach proposed for improving the thermal efficiency of the hydrocarbon thermal conversion system has been to preheat the combustion air by employing the pyrolysis product stream which leaves the pyrolysis reactor at high temperatures, e.g., 1200.degree. to 2000.degree. F. Thus, Bergstrom et al. in U.S. Pat. No. 3,283,028 have disclosed a pyrolysis reactor of special construction which provides for passage of cool air into the apparatus in indirect heat exchange with the hot conversion products after which it is used as combustion air for the fuel to the reactor. These patentees are therefore not teaching the use of low level temperature waste heat streams for air preheat. Belgian Pat. No. 819,761 (Equiv. U.S. Pat. No. 3,980,452) concerns steam reforming in which the hot product gases are used to preheat combustion air; the latter is then passed to an air preheater where it is heated further by exchange with flue gases.
Wiesenthal, in his U.S. Pat. No. 3,426,733, is essentially concerned with a furnace for heating hydrocarbons in which he uses a portion of the feed stream, which is assumed to be already at elevated temperature, for combustion air preheating, then uses the cooled stream to extract heat from the flue gases. In FIG. X, which is the only embodiment suggested for carrying out a chemical process in the furnace, the entire feed stream is first heated in the convection section of the furnace, then is used for combustion air preheating, then is passed through the convection coil and finallly through the radiant heating coil of the furnace. Wiesenthal, in his U.S. Pat. No. 3,469,946, circulates a heat transfer fluid in a closed loop between the convection section and the combustion air, collecting heat in the former and donating this heat to the combustion air.
Hepp in U.S. Pat. No. 2,750,420 uses three pebble heat exchangers in which the pebbles flow downwardly by gravity and at the bottom are hoisted up to the top. The pebbles directly contact successively: the hot pyrolysis effluent gas; combustion air for the furnace; incoming hydrocarbon feed, so that in effect the pebbles quench the pyrolysis products and heat taken up thereby serves as combustion air preheat and as feed preheat. The contacting of the pebbles with pyrolysis products which contain reactive unsaturated monomers and then with air is undesirable since the two are incompatible; also the refractory material can act as a catalyst for polymerization of the monomers and/or as a catalyst for undesirable further cracking which impairs selectivity to valuable components.